Mutagen-sensitive mutants defective in DNA repair mechanisms are collected in Drosophila melanogaster. The mutants are characterized cytogenetically in order to gain a basic understanding of the genetic control of sensitivity to mutagenic agents. The tests used in the initial characterization of these mutants include genetic and cytogenetic mapping, complementation analysis, tests for sensitivity to unrelated mutagens, and tests for pleio-tropic effects on related functions such as recombination. At the present time a fine structure map of the mei-41 region is being constructed in order to ascertain the allelism relationship of mus 104 and mei-41 as well as to confirm the large size of mei 41 found during mutational analysis.